A LAN is a high-speed network that supports many computers connected over a limited distance (e.g., under a few hundred meters). A Virtual Local Area Network (VLAN) is mechanism by which a group of devices on one or more LANs that are configured using management software so that they can communicate as if they were attached to the same LAN, when in fact they are located on a number of different LAN segments. VLAN identifiers (VIDs) or tags, as defined in IEEE standard 802.1Q, are bit-fields that specify the VLAN to which a frame belongs. For example, the use of a service provider tag as part of the Ethernet frame header provides differentiation between traffic flow, i.e., separate service or customer instance. The newly proposed IEEE 802.1ad Provider Bridge specification defines an approach often referred to as “Q-in-Q” because it “stacks” VLAN tags to allow not only customer separation, but differential treatment of customer traffic through the service provider cloud. Additionally, the IEEE 802.1ah Provider Backbone Bridge (PBB) standard defines an approach of encapsulating an end-user's Ethernet frame inside a service provider media access control (MAC) address header and using a 24-bit service instance identifier (I-SID), thereby providing support for a much larger number of service instances. The IEEE 802.1ah standard thus allows for “MAC tunneling” encapsulation and bridging of frames across a provider backbone bridged network.
Although IEEE 802.1ah currently specifies the connectivity to/from 802.1ad bridges via the so-called S-tagged (provider VLAN) service interface, it does not specify how to connect 802.1ah bridges directly to CE devices that run IEEE 802.1Q via the so-called C-tagged (customer VLAN) service interface. Instead, clause 23.4 of version d2-4 of the 802.1ah specification describes support of “a C-Tagged interface for attachment to an 802.1Q C-VLAN network by placing an 802.1ad bridge in front of the 802.1ah bridge. The resulting component connections are Provider Backbone Bridge to Provider Bridge to Customer Bridge.” The problem, however, is that supporting a C-tagged interface in an 802.1ah bridge in this manner is incompetent